1. Field of the Invention
The present invention is directed to the fractionation of polymers based on differing crystallizability of the polymer species using the temperature rising elution technique. More specifically, it involves an improvement in the crystallization procedure. The improvement, which results in increased fractionation efficiency, i.e. improved separation of the polymer species, involves crystallizing the polymer from a dilute polymer solution outside of the column and in the absence of column packing material. The polymer crystallized in this manner is subsequently combined with a suitable column packing material, transferred to the column and eluted in accordance with established temperature rising elution fractionation procedures.
2. Description of the Prior Art
Eluting polymers from a packed column at different temperatures to achieve separation based on crystallinity was first described by Desreux et al, Bull. Soc. Chim. Belg., 59:476 (1950). The term "Temperature Rising Elution Fractionation" (TREF) was first used by Shirayama et al, J. Poly. Sci A-2, 3:906 (1965) to describe the method when employed to fractionate low density polyethylene based on the degree of short-chain branching. While TREF has been applied on a limited basis to amorphous polymers, the technique is now almost exclusively used for crystalline and semi-crystalline polymers. With the applicability of the procedure to new polymer systems and with the development of improved detection capabilities, the use of TREF has evolved from a laboratory curiosity to a sophisticated polymer analytic tool used for the routine evaluation of a variety of polymers. Interest in the technique for preparative purposes has also increased significantly in recent years.
Polymers are a composite of molecular species of varying molecular weight, chain-length, branching, crosslinking, comonomer distribution and the like. Since each molecular species influences the properties of the polymer, it follows that if the effect each species exerts on particular polymer properties can be determined, then trends can be identified which will assist the polymer scientist in designing polymers with enhanced performance characteristics. While there are a variety of procedures available to identify the molecular weight of the various molecular species, i.e. determine molecular weight distribution, techniques which can identify differences between these molecular species based on branching and other structural features are much in demand. In this regard, TREF has become increasingly important since it has the ability to separate molecular species based on any feature which exerts an influence on crystallizability.
Reported TREF procedures have entailed dissolving a crystalline or semi-crystalline polymer in an organic solvent and cooling the polymer solution in the presence of a column packing material. The precipitation/crystallization is typically carried out on the column. Whereas early workers paid little attention to the cooling step and generally used rapid precipitation or natural cooling, more recently the rate of cooling and polymer deposition have been found to significantly influence the subsequent fractionation. The crystallized polymer is then eluted from the column utilizing a suitable solvent as the temperature is raised, either incrementally or continuously at a fixed rate. In preparative TREF, the various fractions are separated and recovered for further evaluation whereas in analytical TREF the polymer is continuously monitored using an in-line detector. Typically for preparative TREF, polymer sample sizes range from about one to ten grams while in analytical TREF the polymer samples are generally less than 0.5 grams.
References which describe various analytical (A) and preparative (P) TREF procedures for the fractionation of polymers are as follows:
Shirayama et al, Supra--(P) PA1 Wijga et al, Makromol. Schem. 36:115 (1960)--(P) PA1 Bergstrom et al, J. Appl. Polym. Sci 23:163 (1979)--(P) PA1 Wild et al, Polym. Preprint A.C.S. 18:182 (1977)--(A) (P) PA1 Wild et al, J. Polym. Sci. Polym. Phys. Ed., 20:441 (1982)--(A) (P) PA1 Usami et al, Macromols, 19:2722 (1986)--(A) (P) PA1 Kelusky et al, Polym. Eng. Sci. 27:1562 (1982)--(A) (P) PA1 Hazlitt et al, U.S. Pat. No. 4,798,081 (1989)--(A) (P) PA1 Nakano et al, J. Appl. Polym. Sci., 26:4217 (1981)--(P) PA1 Kulin et al, Pure & Appl. Chem., 60:9:1403 (1988)--(P) PA1 Mirabella et al, J. Polym. Sci. Polym. Phys. Ed., 25:777 (1987)--(P) PA1 Karoglanian et al, Am. Chem. Soc. Proceedings Pol. Mat. Sci. and Eng. 61:748 (1989)--(A) (P) PA1 Kakugo et al, Macromols 21:2309 (1988)--(P) PA1 Knobeloch et al, SPE Polyolefins IV Conference Prepr. 427 (February 1984)--(A)
While it has been observed with preparative TREF that the ability to cleanly separate the various polymer species has increased over the years due to process improvements, a lower temperature "tail" is still observed for the various fractions obtained indicating an incomplete separation of the species. This "tail" is especially significant at the lower elution temperatures and is believed to be associated with the influence of the column packing material, either in the crystallization and/or in the elution steps. This effect may also be associated with scale-up in the sample size.
It would be highly advantageous if a procedure were available, adaptable to both preparative and analytical TREF, which would minimize this "tailing"; in other words, which would give more efficient crystallization and elution. It would be even more advantageous if the procedure made it possible to conduct preparative TREF runs in shorter periods of time. By shortening the length of time required for collection of the samples, the amount of the solvent used would also be significantly reduced. These and other advantages are realized with the improved process of the present invention.